1. Field of the Invention
The present invention relates to a device for supplying a breathing gas containing a predetermined concentration of a specific additive gas to the lungs of a respirating subject of the type having an inspiration tube through which breathing gas and additive gas are delivered to the lungs of the subject, an expiration tube through which expired gas is carried away from the lungs, a first adjustable gas source for supplying breathing gas to the inspiration tube and a second adjustable gas source for supplying at least the additive gas to the inspiration tube.
2. Description of the Prior Art
In the diagnosis of pulmonary function and treatment of the lungs, having a patient spontaneously breathe a breathing gas containing a predetermined concentration of a specific additive gas may be appropriate. The additive gas could be, e.g., nitric oxide (NO), nitrous oxide (N.sub.2 O), or sulphur hexafluoride (SF.sub.6). For example, NO could be used for relaxing the smooth muscle cells in blood vessels and bronchi or for determining the diffusion capacity of the lungs. N.sub.2 O could be used for determining the blood's rate of flow through the lungs, and SF.sub.6 could be used for determining the functional residual capacity (FRC) of the lungs. Measurement of pulmonary pressure also supplies a measure of the resistance to flow through the lungs.
Certain additive gases, such as NO, react strongly with, e.g., oxygen and must therefore be added to breathing gas close to the patient so the additive gas is fed down into the lungs without having time to combine chemically with oxygen and form a toxic gas NO.sub.2. In order to simplify the supplying of small amounts, such as one or two ppm, of an additive gas, the additive gas is usually mixed with some other gas, such as N.sub.2, in a higher concentration, e.g. 100 ppm. This gas mixture is then added to the flow of breathing gas at a flow rate which is a predetermined fraction of the flow of breathing gas in order to produce the correct concentration. Gas is thus supplied in two stages.
European Application 0 570 612 describes a ventilator which can control very small flows. An accurate concentration of NO or some other additive gas can be delivered to a patient's lungs with such a ventilator, however, the ventilator is complex.